Given that (in the U.S., anyway) cheese-flavored goldfish-shaped crackers are a common snack, it is possible that this is yet another reference to Cheese; that is, showing up between Agatha and Gil when they first meet, it represents the precariousness of their relationship... and being a real goldfish, it represents the vitality of their relationship as well. Finally, Zoing! sees it as a threat, much as a childhood creation (along the lines of Puff, the Magic Dragon) would see a marriage partner for Gil (especially as demanding as a relationship with Agatha would be) as a threat to its relationship with Gil.